Global Magnetospheric Response to an Interplanetary Shock: THEMIS Observations

. We investigate the global response of geospace plasma environment to an interplanetary shock at (cid:24) 0224 UT on May 28, 2008 from multiple THEMIS spacecraft observations in the magnetosheath (THEMIS B and C) and the mid-afternoon (THEMIS A) and dusk magnetosphere (THEMIS D and E). The interaction of the transmitted interplanetary shock with the magnetosphere has global effects. Consequently, it can affect geospace plasma signiﬁcantly. After interacting with the bow shock, the interplanetary shock transmitted a fast shock and a discontinuity which propagated through the magnetosheath toward the Earth at speeds of 300 km/s and 137 km/s respectively. THEMIS A observations indicate that the plasmaspheric plume changed signiﬁcantly by the interplanetary shock impact. The plasmaspheric plume density increased rapidly from 10 to 100 cm (cid:0) 3 in 4 min and the ion distribution changed from isotropic to strongly anisotropic distribution. Electromagnetic ion cyclotron (EMIC) waves observed by THEMIS A are most likely excited by the anisotropic ion distributions caused by the interplanetary shock impact. To our best knowledge, this is the ﬁrst direct observation of the plasmaspheric plume response to an interplanetary shock’s impact. THEMIS A, but not D or E, observed a plasmaspheric plume in the dayside magnetosphere. Multiple spacecraft observations indicate that the dawn-side edge of the plasmaspheric plume was located between THEMIS A and D (or E).


Introduction
The interaction of interplanetary shocks (usually fast forward shocks) with the magnetosphere includes several phases, including interaction with the bow shock, transmission through the magnetosheath, interaction with the magnetopause, transmission into the magnetosphere as fast and intermediate mode waves, modifications of the field-aligned and ionospheric current systems, and perturbations in ground magnetograms (Samsonov et al., 2007).The interaction of interplanetary shocks with the bow shock has been extensively studied (e.g., Shen and Dryer, 1972;Grib et al., 1979;Zhuang et al., 1981;Samsonov et al., 2006Samsonov et al., , 2007;;Zhang et al., 2009).In MHD simulations, the interaction of an interplanetary shock with the bow shock launches a fast shock into the magnetosheath and creates a new discontinuity (Zhuang et al., 1981) where the magnetic field strength and density increase, the temperature decreases and the velocity remains unchanged (Samsonov et al., 2006).The transmitted fast shock and new discontinuity have been observed ( Šafránková et al., 2007;Přech et al., 2008).
Past work has predicted that the interaction of an interplanetary shock marked by a pressure increase with the bow shock results in earthward then sunward motion of the bow shock.By analyzing the Rankine-Hugoniot conditions, Grib et al. (1979) and Völk and Auer (1974) predicted that the bow shock moves towards the magnetosphere after interaction with the interplanetary shock.Then the interaction of the transmitted fast shock and the magnetopause (considered a tangential discontinuity) results in a fast rarefaction wave propagating toward the bow shock.This rarefaction wave could result in outward bow shock motion.In MHD simulations, the bow shock begins moving earthward immediately after an encounter with an interplanetary shock at velocities of ∼100 km/s (Samsonov et al., 2006).Results from a three-dimensional magnetosheath numerical model show that both a fast reverse shock and a fast expansion wave (rarefaction wave) may result from the interaction of the interplanetary shock with the magnetopause depending on boundary conditions of the model (Samsonov et al., 2006).The existence of the rarefaction wave reflected from the magnetopause due to the shock-magnetopause interaction was confirmed by a case study employing observations made by Cluster spacecraft in the magnetosheath (Maynard et al., 2008).Based on results from global MHD simulations, Samsonov et al. (2007) suggested that the dayside ionosphere reflects the transmitted fast shock and that the bow shock and the magnetopause move sunward when the reflected fast shock passes.Earthward then sunward bow shock motion due to the interaction of an interplanetary shock with the bow shock has been observed (e.g., Šafránková et al., 2007).Šafránková et al. (2007) concluded that the observed bow shock crossings result from the interplanetary shock-magnetosphere interactions because there are no further changes in the upstream dynamic pressure or IMF that could cause them.
Enhanced solar wind pressure behind a shock front compresses the magnetopause, resulting an enhancement of the Chapman-Ferraro current at the magnetopause.This interaction launches a fast magnetosonic wave that propagates to the ionosphere.Typically, low-latitude magnetometers record rapid increase in the northward magnetic field, called sudden impulses (SI), or storm sudden commencements (SSC) if a geomagnetic storm follows (Tamao, 1975;Araki, 1977;Huttunen et al., 2005).In addition to the inward propagation there is a field-aligned propagation of Alfvén waves due to the shock impact, changing the ionospheric currents, and causing auroral activity (Zhou and Tsurutani, 1999).
Electromagnetic ion cyclotron (EMIC) waves are generated by the ion temperature anisotropy (T ⊥ > T ∥ ) (e.g., Cornwall, 1965).The frequencies of EMIC waves are below the local proton gyrofrequency.In the magnetosphere, the frequency of the EMIC waves ranges from 0.1 to 5 Hz.
The plasmasphere is a region located in the dipolar portions of the Earth's magnetosphere and populated by cold (∼eV) and dense plasma of ionospheric origin (Lemaire and Gringauz, 1998).
They found that plasmaspheric plumes were observed mostly for moderate Kp and not for small Dst.They also showed that plumes are mainly located in the afternoon and pre-midnight MLT sectors.Plasmaspheric plumes have been suggested as a major cause of EMIC waves (Fuselier et al., 2004).
Most of the previous studies on EMIC waves and plasmaspheric plumes were during magnetic storms or substorms.In this paper, we investigate the global response of geospace plasma environment to an interplanetary shock from multiple THEMIS spacecraft observations in the magnetosheath (THEMIS B and C) and the mid-afternoon (THEMIS A) and dusk magnetosphere (THEMIS D and E).THEMIS A observed accelerated plasmaspheric plume population in the mid-afternoon magnetosphere when the plasma flow was ∼50 km/s.Meanwhile, THEMIS D and E did not observed plasmaspheric plume population in the dusk magnetosphere.These observations indicate that dawn-side edge of the plasmaspheric plume was located between THEMIS A and D (or E).
The plume properties changed significantly after the shock arrival.The density increased from 10 to 30 cm −3 and the ion distribution changed from isotropic to strongly anisotropic.THEMIS A also observed EMIC waves which were most likely excited by the anisotropic ion distributions caused by the interplanetary shock impact.

Spacecraft Observations
The plasma observations reported in this paper were obtained with Electrostatic Analyzer (ESA) (McFadden et al., 2008b) on the THEMIS spacecraft (Angelopoulos, 2008).In one 3-s spin, ESA measures the 3-D ion and electron distributions over the energy range from a few eV up to 30 keV for electrons and 25 keV for ions.The magnetic field observations presented herein are obtained by the Fluxgate Magnetometer (FGM) (Auster et al., 2008) which measures the DC magnetic field up to 128 Hz.
Figure 1 shows an interplanetary shock observed by the WIND spacecraft located at (x,y,z)= (257, 52, 23) GSE R E .The top three panels show plasma moments measured by SWE (Ogilvie et al., 1995).The fourth panel shows the calculated dynamic pressure.The bottom two panels show magnetic fields measured by MFI (Lepping et al., 1995).The IMF was southward from 0100 to 0112 UT and from 0118 to 0125 UT, mainly northward from 0112 to 0118 UT and after 0125 UT.The vertical dashed red line at 01:17:38 UT marks the interplanetary shock crossing.The interplanetary shock is a fast forward shock which is characterized by increases in the solar wind density, thermal temperature, bulk velocity, dynamic pressure (nmv 2 ) and magnetic field strength.perature.This shock was followed by a discontinuity at 0224:04 UT which is characterized by a density increase and a temperature decrease.This is the new discontinuity predicted by MHD theory (Samsonov et al., 2006).Both discontinuities propagated Earthward towards THEMIS C. The separation between the shock and discontinuity observed by THEMIS C at 0224:16 UT and 0225:42 UT is larger than that observed by THEMIS B at 0223:47 UT and 0224:04 UT due to the greater propagation speed of the shock than the discontinuity.The speed of the shock is 300 km/s and the speed of the discontinuity is 137 km/s (from R-H conditions and the timing method).The dynamic pressure increases associated with both discontinuities then compressed the magnetosphere.THEMIS D, E and A inside the magnetosphere observed antisunward-moving plasmas beginning at almost the same times (marked by black dashed lines in the bottom 6 panels), and lasting for at least 2 minutes.the shock passage at 0225 UT, the density was 10 cm −3 , indicating that THEMIS A observed a plasmaspheric plume.The density increased to 30 cm −3 at 0226 UT and 100 cm −3 at 0229 UT.We expect the arrival of an interplanetary shock to result in a significant brightening in emissions in the outer magnetosphere which can be detected by EUV imagers.The density drop from 5 to 0.5 cm −3 observed by THEMIS A at (3.6, 8.1, -1.3) GSM R E indicates the dusk-side edge of the plume at 0300 UT.
The EMIC wave activity seems to be closely related to the accelerated plasmaspheric plume population from the fourth and fifth panels of Figure 4. Ion temperature anisotropy can stimulate EMIC waves in frequency ω/Ω i < A i /(1 + A i ) (Horne and Thorne, 1993), where Ω i is the ion gyrofrequency, A i is the ion temperature anisotropy which is defined by The bottom two panels of Figure 4 present ion distributions for 3 s time intervals before (left) and after (right) the shock arrival.The ion distribution before the shock arrival was nearly isotropic, while it was strongly anisotropic towards perpendicular temperatures above 50 eV after the shock passage.The displacement towards positive V ⊥ in the right panel is evidence for accelerated flows.The anisotropy A i is ∼ 1 at 0243:45 UT, and the EMIC wave frequency should be less than 0.5 Hz which is consistent with the wave frequency shown in the fourth panel of Figure 4. Therefore, the observed EMIC waves were most likely excited by the anisotropic ion distributions caused by the interplanetary shock impact.To our best knowledge, this is the first direct observation of the plasmaspheric plume response to an interplanetary shock's impact.The magnetic field strength compression ratio of this IP shock is 2 which is close to the average compression ratio of interplanetary shocks for both solar maximum (1.97) and solar minimum (1.93) (Echer et al., 2004), therefore, this event is common.
With multiple THEMIS spacecraft, the spatial distribution of plasmaspheric plumes can be estimated.The bottom six panels in Figure 3 show that while THEMIS A observed accelerated plasmaspheric plume population at (3.8, 7.5, -1.1) GSM R E (16 MLT, L ≈ 8.5) from 0225 to 0226 UT, THEMIS D and E at (-0.7, 11.4, 0.6) GSM R E and (0.3, 11.6, 0.2) GSM R E (18 MLT, L ≈ 11.6) did not observed plasmaspheric plume population during this time interval.This indicates that the dawn-side edge of the plasmaspheric plume was located between THEMIS A and D (or E) as illustrated in Figure 5. THEMIS A (D, E) was located in the L-MLT bin where there is a high (low) probability to observe a plasmaspheric plume as shown in Figure 8 of Darrouzet et al. (2008).The Kp index was 3 during this time interval which is consistent with Darrouzet et al. (2008) that plumes were mostly observed during moderate Kp (3-6).However, the Dst was only -5 which is inconsistent with Darrouzet et al. (2008) that plasmaspheric plumes were never observed for small Dst.

Conclusions
The global magnetospheric response to an interplanetary shock has been investigated using the THEMIS spacecraft observations.With THEMIS B and C in the magnetosheath, THEMIS A in the mid-afternoon magnetosphere, and THEMIS D and E in the dusk magnetosphere, the THEMIS spacecraft offer a remarkable opportunity to track the propagation of the shock and the magnetospheric response.The interaction of the transmitted interplanetary shock with the magnetosphere has global effects.Consequently, it can affect geospace plasma significantly.
The main conclusions of this paper can be summarized as follows: 1.The interaction of an interplanetary shock with the bow shock launched a fast shock and a discontinuity which propagated toward the Earth at speeds of 300 km/s and 137 km/s respectively.2. THEMIS A, but not D or E, observed a plasmaspheric plume in the dayside magnetosphere.
Multiple spacecraft observations indicate that the dawn-side edge of the plasmaspheric plume was located between THEMIS A and D (or E) as illustrated in Figure 5.
3. The plume properties changed significantly by the interplanetary shock impact.The density increased from 10 to 100 cm −3 in 4 min and the ion distribution changed from isotropic to strongly anisotropic distribution.4. THEMIS A also observed EMIC waves which were most likely excited by the anisotropic ion distributions caused by the interplanetary shock impact.To our best knowledge, this is the first direct observation of the plasmaspheric plume response to an interplanetary shock's impact.

Figure 2
Figure 2 shows the trajectories of the THEMIS spacecraft from 0200 UT to 0300 UT on May 28, 2008.Five different symbols in Figure 2 mark the positions of 5 THEMIS probes at 0300 UT.THEMIS B and C were in the magnetosheath and THEMIS D, E, and A were inside the magnetosphere at 0200 UT.THEMIS D and E were very close to each other on the dusk flank.The IP shock observed by WIND (shown in Figure 1) propagated toward the Earth and was observed by the THEMIS spacecraft.The top four panels in Figure 3 show THEMIS B observations, the following four panels show THEMIS C observations.The bottom six panels show observations by THEMIS D, E and A. The IP shock first reached THEMIS B in the magnetosheath.The first panel shows the plasma flow V x component.The second panel shows the ion density.The third panel shows the ion temperature.The fourth panel shows the ESA plasma ion spectrum.The bow shock moved inward past THEMIS B at ∼0225 UT, as indicated by the transition to low solar wind densities and temperatures but high velocities.The black dashed line at 0223:47 UT marks the transmitted shock which can be identified by increases in the plasma flow speed, density and tem-

Fig. 1 .
Fig. 1.An interplanetary shock observed by the WIND spacecraft upstream at (257, 52, 23) RE.From top to bottom: ion density, thermal velocity, component of the flow velocity Vx along the Sun-Earrth line, dynamic pressure, components of the magnetic fields in GSE coordinates, and the magnetic field strength.The vertical dashed red line at 01:17:38 UT marked the interplanetary shock crossing.

Figure 4 Fig. 2 .
Figure 4 shows THEMIS A observations from 0220 to 0310 UT, May 28, 2008.The first panel shows three components of the magnetic field in GSM coordinates with 0.25 s time resolution.The magnetic field strength (second panel) increased sharply from 60 nT to 75 nT at ∼0225 UT due to the pressure enhancement associated with the IP shock.Then the magnetic field strength decreased slowly to 68 nT at 0234 UT.The magnetic field strength showed a few more compressions and relax-

Fig. 3 .
Fig. 3.The propagation of the interplanetary shock through the magnetosheath and in the magnetosphere.From top to bottom: THEMIS B plasma flow Vx component, ESA ion density, temperature, spectrum, THEMIS C plasma flow Vx component, ESA ion density, temperature, spectrum, THEMIS D plasma flow Vx component, ESA ion spectrum, THEMIS E plasma flow Vx component, ESA ion spectrum, THEMIS A plasma flow Vx component, ESA ion spectrum.The bow shock crossing was observed by THEMIS B near 0225 UT.The vertical black dashed lines mark the transmitted shock (top 8 panels) or the time when the plasma inside the magnetosphere started to move earthward (bottom 6 panels).The vertical blue dashed lines in the top 8 panels mark the discontinuity produced by the interaction of the IP shock and the bow shock.

Fig. 4 .
Fig. 4. THEMIS A (in the magnetosphere) observations of the EMIC wave activity and plasmaspheric plumes.From top to bottom: three components of the magnetic field in GSM coordinates, the magnetic field strength, the x component of the plasma flow velocity in GSM coordinates, wavelet analysis result for the By component of the magnetic field, ESA ion spectrum, plasma density derived from the spacecraft potential, and ion distributions for 3 s time intervals before (left) and after (right) the shock arrival.The thick black lines point toward the sun.

Fig. 5 .
Fig. 5.A cartoon illustrates a plasmaspheric plume observed by THEMIS A (but not observed by THEMIS D and E ) in the mid-afternoon magnetosphere.